Cavity-backed spiral antenna with mode suppression

ABSTRACT

THIS PLANAR UNIDIRECTIONAL CAVITY-BACKED SPIRAL ANTENNA HAS A CYLINDRICAL RESISTANCE CARD IN THE CAVITY COAXIAL WITH THE CENTER CONDUCTOR THEREOF TO SUPPRESS MODES OF PROPAGATION OF ELECTROMAGNETIC WAVES IN THE CAVITY OTHER THAN THE DOMINANT MODE.

n SAMUEL CHUNG-SHU KUO CAVITY-BACKED SPIRAL ANTENNA WITHMODEISUPPRESSION Filed March a, 1969 3 Sheets-Sheet 1 A I///l INVENTOR. SAMUEL CHUNG-SHU KUO AGENT J35." 12, 1971' SAMUEL CHUNG1SHU"KUO ChVITY-BACKED SPIRAL ANTENNA WITH MODE SUPPRESSION 3 SheetsSheet 2 Filed March 5. 1969 INVENTOR. .SAMUEL CHUNG-SHU KUO 7 BY M 4M AGENT 1971 SAMUEL CHUNG-SHU Kuo"'""" CAVIIY-BACKED SPIRAL ANTENNA WITH MODE SUPPRESSION Filed Marcus, 1969 3 Sheets-Sheet 5 zea 35' IE- El I NVENTOR.

SAMUEL CHUNG-SHU KUO AGENT United States Patent Ofltice Patented Jan. 12, 1971 3,555,554 CAVITY-BACKED SPIRAL ANTENNA WITH MODE SUPPRESSION Samuel Chung-Shu Kuo, San Jose, Calif., assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Mar. 3, 1969, Ser. No. 803,595 Int. Cl. H01q 1/36 US. Cl. 343-895 9 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to spiral antennas and more particularly to planar spiral antennas having unidirectional radiation patterns.

The planar equiangular spiral antenna is described in the IRE Transactions on Antennas and Propagation, vol. AP-7, No. 2, April 1959, pp. 181-187. This antenna provides circularly polarized, single lobe, bidirectional radiation, normal to the plane of the antenna. This antenna is particularly useful in airborne direction finding systems since it can be mounted flush with the skin of the aircraft. The bidirectional radiation of the spiral antenna is the principal feature limiting its usefulness. A common technique for obtaining unidirectional radiation from the planar spiral antenna is to back it with a coaxial cavity. In order to obtain broad band operation, the bottom of the cavity may be stepped tomake the height of the cavity backing the active regions of the spirals substantially equal to a quarter-wavelength at the operating frequency. This causes reflected radiation in the cavity and waves radiated in the same direction to be in-phase in the plane of the spirals so that the waves combine constructively. A single four-arm spiral antenna may be utilized in a sum and difference monopulse direction finding system. The two pairs of spirals are fed so that they provide sum and difference mode beams. Signals received while operating in the sum and difference modes are processed to determine the direction of the signal source. In order to obtain accurate direction finding information, the difference between the peak amplitudes of the sum and difference beams must remain constant as frequency is varied. Also, the axial ratio of the antenna beams must remain substantially equal to db as frequency is varied. The axial ratio of the antenna beam is defined as the ratio of the strength of the horizontal electrical field to the strength of the vertical electrical field. Although the planar spiral antenna theoretically has an unlimited bandwidth, the cavity backed spiral has a limited useful bandwidth since the cavity will simultaneously support the dominant mode as well as higher order modes of propagation of electromagnetic waves when the cavity size is large in terms of wavelength. This will cause deterioration of the radiation pattern and a decrease in the antenna gain in the vicinity of cavity resonance. Unless the higher order modes are suppressed, the useful bandwidth is limited to approximately 3 :1 for a two arm spiral and 2:1 for a 4-arm spiral operating in the sum and difference mode. A previous attempt to solve this problem was to back the spirals with a cavity filled with a material that absorbs all the radiation in the cavity. The obvious disadvantage of this technique is the high loss that is involved and the resultant low efliciency of the antenna.

An object of this invention is the provision of an improved spiral antenna providing unidirectional radiation.

Another object is the provision of a broadband planar unidirectional spiral antenna.

SUMMARY OF THE INVENTION The cavity backed spiral antenna embodying this invention has a tubular resistance card in the cavity to prevent simultaneous excitation and propagation of a plurality of modes in the cavity.

DESCRIPTION OF DRAWINGS FIG. 1 is a top view of a unidirectional spiral antenna embodying this invention;

FIG. 2 is a section taken along the line 2r'-2 in FIG. 1;

FIG. 3 is a top view of a second embodiment of this invention;

FIG. 4 is a section along the line 44 of FIG. 3;

FIG. 5 is a section view of a third embodiment of this invention; and

FIG. 6 is a section view of a fourth embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, this antenna comprises four planar equiangular spiral conductive patterns 11, 12, 13 and 14 on a dielectric disc 15. Although equiangular spiral patterns are shown in FIG. 1, they could also be other types of spirals such as the Archimedes spiral. The spiral patterns 11 to 14 may be formed by conventional printed circuit techniques from the copper clad on a Teflon or fiber glass base. In practice, the conductive patterns are approximately 0.005 inch thick. They are shown much thicker than this in the figures, however, for illustrative purposes. Alternatively, the spirals may be cut from a thin sheet of copper and bonded to a dielectric base.

The lengths L of the spiral patterns are representable as d 2 /2 ama dp which reduces to P1= and a(-5) :ICH where p and p are shown in FIG. 1, K is a positive constant which determines the size of the terminal region of pattern 11, 6 is a fixed angle between the radius vectors p and p that determines the width of pattern 11, e is the base of Naperian logarithm, and K is defined as a6 fl K e p1 Thus, the edges of pattern 11 are identical curves with one curve rotated through the fixed angle 8 with respect to the other. It is this rotation angle that gives the spiral a finite width.

The radius vectors defining the inner and outer edges of the other patterns are also defined by Equations 3 and 4 respectively, except that the radius vectors associated with patterns 12, 13 and 14 are advanced by 1r/2, 1r and 31r/2 radians, respectively, over that shown in these equations. The exact values of the angle 6, the arm lengths L and the constants a and k are a function of the desired characteristics from the antenna.

The planar spiral conductive patterns are backed by a cylindrical coaxial cavity 16. The dielectric disc supporting the spirals is rigidly secured to the top 17 of side wall 18 of the cavity by bonding and screws (not shown). The center conductor of the coaxial cavity is a post 19 which is formed, such as by machining or casting, in the bottom wall 20 of the cavity. The post 19 extends over the full height of the cavity.

The spirals are fed by a beam-forming network comprising coaxial transmission lines 21 to 24 and a microwave phase shifter (not shown). The coaxial cables are symmetrical about the center of the antenna and extend through associated bores in post 19. The outer conductors of the transmission lines are electrically connected to post 19 such as by soldering. The inner conductors 21' to 24' of the transmission lines are electrically connected to the ends of the associated patterns 11 to 14, respectively, at the center of the antenna.

The cross section of the coaxial cavity is the same as that of a cylindrical waveguide and will support higher order waveguide modes when the cavity diameter is large with respect to operating wavelength. For this reason, the cavity will simultaneously support the dominant mode and a number of higher order modes of propagation of electromagnetic signals in the cavity at some high frequency. This multimode propagation and cavity resonances causes the antenna radiation patterns to deteriorate. As a result of this operation, this antenna has a useful bandwidth in the order of 2:1.

In accordance with this invention, a tubular resistance card 27 is located in the cavity coaxial with post 19. The resistance card is rigidly secured in a cylindrical groove 28 in the bottom of the cavity by bonding. The resistance card may, by way of example, be a metal film resistance card comprising a thin film 29 of pure metal approximately 50 millionths of a inch thick evaporated on a dielectric substrate 30 such as Mylar. The thicknesses of members 29 and 30 are greatly enlarged in the draw ings for illustrative purposes. Such a resistance card is manufactured by Filmohm Corporation, New York, N.Y. Alternatively, a resistance card comprising a graphite impregnated dielectric sheet may be employed. The wall of tube 27 is normal to the base 20 or the cavity. It is believed that the resistance card introduces a small loss in the cavity that causes suppression of the higher order modes and in this manner prevents deterioration of the performance of the antenna.

An embodiment of this invention for operating over a broad band of frequencies is illustrated in FIGS. 3 and 4. This antenna comprises two planar equiangular spiral conductive patterns 31 and 32 formed on a dielectric disc 34. The spirals are fed by a beam forming network comprising a pair of rigid coaxial transmission lines 35 and 36 (see FIG. 4) which are symmetrically located with respect to the center of the antenna. The center conductors 37 and 38 of the associated transmission lines 35 and 36 are electrically connected to the ends of patterns 31 and 32, respectively, adjacent the center of the antenna.

Spirals 31 and 32 are backed by a cylindrical coaxial cavity 41 having a center conductor that is a post 42. Cylindrical steps 43 and 44 are formed in the bottom 45 of the cavity, such as by machining or casting, coaxial with post 42. Feed lines 35 and 36 extend through bores 47 and 48, respectively, in post 42 and the bottom of the cavity. The outer conductors of the feed lines are electrically connected to the center post 42 and wall of the cavity such as by soldering.

In operation, only small portions of spirals 31 and 32 comprise the active regions of the antenna at a particular operating frequency. As the frequency is increased, the active regions shift toward the center of the antenna. The heights, diameters, and number of steps in the cavity are selected so that the heights of the cavity under the active regions of the spirals are approximately one-quarter wavelength at the operating frequencies of the antenna. This causes radiation in the cavity that is reflected back to the plane of the spirals to be in-phase with and constructively combine with radiation for the spirals.

Cylindrical sleeves 50 and 51 of resistance card are located in the cavity to prevent excitation and propagation of higher order modes in the cavity. The sleeves are supported on shoulders 54 and 55 of the associated steps 43 and 44 to which they are secured by bonding and are oriented normal to the bottom of the cavity. The heights and diameters of sleeves and 51 are determined empirically.

A section view of an alternate embodiment of this invention for operating over a broad band of frequencies is illustrated in FIG. 5. The spirals in FIG. 5 are the same as those in FIGS. 3 and 4. Primed reference characters refer to similar components in FIGS. 3 and 4. The cavity 41 backing spirals 31 and 32' has a conically shaped bottom 45 that causes the height of the cavity to vary linerally from a maximum at side wall 46 to a minimum determined by step 57. The cylindrical sleeves 50 and 51 of resistance card are rigidly secured in cylindrical grooves in the bottom of the cavity by bonding.

A section view of another embodiment of this invention for operating over a broad band of frequencies is illustrated in FIG. 6. The spirals in FIG. 6 are also the same as those in FIGS. 3 and 4. Double primed reference characters refer to similar components in FIGS. 3 and 4. The height of the cavity 41" backing spirals 31" and 32" is periodically varied abruptly by steps 43 and 44" therein. The resistance card sleeves 50 and 51" extend over the full height of the associated steps. Sleeves 50" and 51" each extend the same distance above the height of the bottom 45" of the cavity.

By way of example, an antenna similar to that illustrated in FIG. 6 that was actually built and tested had the following dimensions and characteristics.

Spiral patterns 31" and 32":

Length L37 inches 660 k-0.045 inch a0.093

Cavity 41":

Diameter3.5 inches Height0.8 inch Post 42" diameter0.30 inch Step 43":

Diameter2.0 inches Height0.35 inch Step 44":

Diameter1.1 inches Height0.55 inch Post 42":

Diameter--0.30 inch Resistance card 50":

Height-07 inch Loss377 ohms/square Resistance card 51":

Height-0.75 inch Loss377 ohms/square This antenna operated over a frequency band of greater than 10:1 and had a voltage standing wave ratio of less than 3:1, an axial ratio of less than 2 db, and a gain at X band frequencies of greater than 5 db.

What is claimed is:

1. A unidirectional planar spiral antenna comprising a planar sheet of dielectric material,

a pair of conductors associated with said dielectric sheet, said conductors having configurations defining planar spirals,

means for coupling electrornoagnetic wave signals to and from said spiral conductors for causing the antenna to provide bidirectional radiation patterns,

5 a conductive cavity having an opening in one end thereof, said cavity supporting said dielectric sheet in the opening for backing said spiral conductors in one of the directions of radiation for causing the antenna to provide a unidirectional radiation pattern, and a tubular resistance card supported in the bottom wall of said cavity that is opposite said spiral conductors, said card having an opening in the end thereof proximate said spiral conductors.

2. The antenna according to claim 1 wherein said tubular resistance card has a wall that is substantially perpendicular to the plane of said spiral conductors.

3. The antenna according to claim 2 wherein the top of said cylindrical resistance card is spaced from said spiral conductors.

4. The antenna according to claim 2 wherein said cavity is a coaxial cavity having a center conductor and said tubular resistance card is coaxial with said center conductor.

5. The antenna according to claim 4 wherein the height of said cavity between the bottom wall thereof and the plane of said spiral conductors is varied for extending the bandwidth of the antenna and wherein said tubular resistance card has a circular cross section in a plane parallel to the plane of said spiral conductors.

6. The antenna according to claim 5 including a cylindrical step in the bottom of said cavity for providing the change in the cavity height, said resistance card being contiguous with at least a portion of the circumference of said step and extending into the region between the top of said step and the plan of said spiral conductors.

7. The antenna according to claim 6 including a plurality of said steps in the bottom of said cavity and a cylindrical resistance card associated with each of said steps.

8. The antenna according to claim 7 wherein each of said resistance cards is contiguous with the circumference of the associated step over the height of the latter.

9. The antenna according to claim 8 wherein the top of each of said resistance cards is spaced the same distance from the plane of said spiral conductors.

References Cited UNITED STATES PATENTS 3,192,531 6/1965 Cox et a1. 343895 3,358,288 12/1967 Dubost et a1. 343-895 3,441,937 4/1969 Clasby et al. 343789 ELI LIEBERMAN, Primary Examiner US. Cl. XJR. 333*83; 343-789 

